I believe any radial fires in the order (for a 9-cylinder)
1-3-5-7-9-2-4-6-8
In other words the pistons firing follows the crankshaft round. Being a 4-stroke the crankshaft has to go round twice for each set of firing, and the firing order follows the con rod crank round.
It's for this reason there are always an odd number of cylinders on a 4-stroke radial.